One of the important issues in which the automobile industry has always been engaged is to reduce the weights of automobile bodies while maintaining the strengths thereof and to thereby enhance the fuel economy of the automobiles in order to reduce the amount of CO2 emission from the viewpoint of global environmental protection.
For reducing the weights of automobile bodies while maintaining the strengths thereof, it is effective to increase the strength of steel sheets used as a material of automotive parts and thereby reduce the thickness of the steel sheets. Since automotive parts are commonly formed into shape by press working, burring, or the like, the high-strength steel sheets used as a material of automotive parts are to have excellent formability in addition to a desired strength.
Recently, high-strength steel sheets having a tensile strength of more than 1180 MPa have been increasingly used as a material of automobile body frames. Since such high-strength steel sheets are generally bent when formed into shape, the high-strength steel sheets are to have excellent bending workability. Furthermore, such materials of automobile body frames are to have resistance to deformation, that is, a high yield strength, in order to ensure the safety of occupants at the time of impact or the like. It is particularly difficult to address the above issues in the case where the high-strength steel sheet is a hot-dip galvanized steel sheet. In order to meet the above-described requirements, for example, Patent Literature 1 discloses a technique concerning a steel sheet having a strength of about 1180 MPa which has a high yield strength and excellent bending workability.
In the technique disclosed in Patent Literature 1, the C content in the steel sheet having a tensile strength of about 1180 MPa is high, that is, specifically, 0.16% or more. Therefore, it is not considered that the steel sheet having a tensile strength of about 1180 MPa disclosed in Patent Literature 1 has sufficient spot weldability, which is one of the important properties for steel sheets for automobiles.
Patent Literature 2 discloses a technique concerning a high-strength steel sheet having a low C content, that is, specifically, 0.15% by mass or less. Although the high-strength steel sheet disclosed in Patent Literature 2 has good spot weldability and good bending workability, the yield strength of the high-strength steel sheet disclosed in Patent Literature 2 is low, that is, specifically, less than 780 MPa. In Patent Literature 2, any approach to increase the yield strength of the high-strength steel sheet is not studied.
[PTL 1] Japanese Unexamined Patent Application Publication No. 2012-237042
[PTL 2] Japanese Unexamined Patent Application Publication No. 2011-132602